Well who is the husband paying attention to when he's wanking to a porn video? Not his wife that's for sure.
I don't need my husband to pay attention to me all the freakin time. Some men have very high sex drives. I don't think many women would want their husband after them like that all the time. So this is a way of dealing with that drive. Now, if the man is using porn INSTEAD of being intimate with the wife, that's another story and that's not what I'm talking about here.